1. Field of the Invention
The present invention relates to a transmission system for a vehicle and more particularly to an automatic transmission whose gear trains are originated from those of a conventional manual transmission.
2. Discussion of Prior Art
Generally, a manual transmission, in which the gear is manually shifted, has an input shaft directly connected to an engine and including a plurality of drive gears and has an output shaft including a plurality of driven gears paired with the corresponding drive gears and connected to drive wheels. That is, there are provided a plurality of shift gear trains between the input shaft and the output shaft. When a gear is changed, after a clutch is disengaged, changeover mechanisms such as a synchromesh mechanism are manually operated to change over paired shift gear trains and then the clutch is engaged. This sequence of manual operations accomplishes a gear shift of a vehicle.
The manual transmission can be converted into an automatic transmission by replacing the sequence of manual operations with automatic operations using hydraulic actuators. This type of automatic transmission has advantages such as a small number of components and has a good transmission efficiency of power and the like, compared to a conventional automatic transmission primarily constituted by planetary gears, friction engagement elements (clutches, brakes) and the like.
This type automatic transmission having a plurality of shift gear trains is called an Automated Manual Transmission (hereinafter, referred to as xe2x80x9cAMTxe2x80x9d). Japanese Patent Application Laid-open No. Toku-Kai 2000-55184 discloses an AMT including a main clutch (dry type clutch) for changing over the connection of a crank shaft with an input shaft between an engagement condition and a disengagement condition and a bypass clutch (hydraulically operated multiple disc clutch) for transmitting torque from the input shaft to an output shaft to prevent a so-called xe2x80x9ctorque dropxe2x80x9d. When the main clutch changes an engagement condition, the bypass clutch is engaged by hydraulic pressure so as to prevent an abrupt drop of output torque when the gear is shifted and thus a shift shock can be alleviated.
However, a prior art dry clutch type AMT has difficulty generating such a positive and continuous creep for a long time as obtainable in a conventional torque converter type automatic transmission. As a result, this type transmission provides a driver with an awkward drive feeling different from that experienced with a conventional automatic transmission vehicle when a vehicle travels at very low speeds, such as when parking, starting on grades and the like. In order to generate a creep in AMT and to eliminate such awkwardness from AMT, it is necessary to positively apply a slip control to the main clutch of AMT for engaging or disengaging a crank shaft with an input shaft. However, in designing the main clutch, it is very difficult to satisfy both characteristics, a characteristic for smoothly transmitting power of an engine to the input shaft and a characteristic for enduring continuous slippage.
Further, a complicated slip control of the transmission system and some forced cooling device are required, this increasing the manufacturing cost of the transmission.
In a case where a conventional torque converter is incorporated into a clutch housing for a manual transmission vehicle, it is easy to obtain a creep effect and a torque amplitude effect but a continuous delivery of drag torque of the torque converter incurs failures such as gear clash unless the torque delivery is shut off at gearshifting.
Japanese Patent Application Laid-open No. Toku-Kou-Shou 53-22215 discloses a semiautomatic transmission in which a manual transmission is equipped with a torque converter. The semiautomatic transmission is provided with a friction clutch for delivering or shutting off power of an engine between the output side of the torque converter and the input side of the transmission. When shifting gears, the friction clutch is released based on a signal from a shift lever and then synchromesh mechanisms are changed over. In this case, since it is difficult to install a hydraulically controlled lock-up clutch for the structural reason, the torque converter is always in a slip condition and accordingly improved fuel economy can not be expected. Further, this transmission has disadvantages in a shift shock which occurs when the friction clutch is reengaged and a shift time elongated by the release and reengagement actions of the friction clutch.
It is an object of the present invention to provide an automatic transmission having a plurality of shift gear trains capable of preventing a torque drop when the gear is shifted. It is another object of the present invention to provide an automatic transmission having a plurality of shift gear trains, capable of regulating transmission torque according to vehicle running conditions.
To achieve the objects, the automatic transmission system for a vehicle having a plurality of drive gears mounted on an input shaft, a plurality of driven gears mounted on an output shaft and meshing with the drive gears, a torque converter disposed between a crankshaft of an engine and the input shaft, synchromesh mechanisms for synchronously engaging the drive gears with the driven gears and a shift controller for automatically actuating the synchromesh mechanisms so as to obtain a required gear ratio, includes a lock-up clutch incorporated in the torque converter for connecting a turbine shaft of the torque converter with the crankshaft, an electronically controlled throttle valve for automatically operating to reduce a rotation speed of the crank shaft when the gear is shifted, a bypass clutch for transmitting torque from the input shaft to the output shaft when the gear is shifted while the electronically controlled throttle valve operates to reduce a rotation speed of the crankshaft, and an input clutch provided between an output element of the torque converter and the input shaft for selectively controlling a torque transmission from the crankshaft to the input shaft when the gear is shifted.